The use of high powered lasers as weapons is known. One of the major problems encountered with high powered laser beams in the atmosphere is an effect known as “thermal blooming”. The effect is a result of the heating of the atmospheric elements along the path of the beam by the absorption of a small amount of the laser energy by the elements in the atmosphere. The heating causes differential phase changes at incremental positions along the beam path, which in turn causes the laser energy to become more widely dispersed. The heating and therefore the distortion of the beam is directly in the path of the laser beam and can be quite severe. The result is that at a target, the power per unit area (fluence) is smaller than that required to do the requisite amount of damage.
Continuous wave (CW) lasers suffer the most from thermal blooming, and therefore laser weapons generally are pulsed, with the pulse length chosen to be short compared with the time required for the thermal blooming distortion in the atmosphere.
When a train of pulses is used, thermal blooming caused by residual heating of previous laser pulses in the train creates what is called “overlap blooming”. The repetition rate for a train of pulses is limited by the wind clearout time (the time required for cooling adequately between pulses) which can be determined to be the beam diameter, D, divided by the transverse wind velocity, V. In the prior art, the repetition rate for a train of laser pulses is on the order of three pulses per wind clearout time, 3/D/V.
If each pulse is made short enough, more fluence can be delivered with less blooming by each individual pulse. Increasing the power of each pulse is beneficial up to the point where there is gas breakdown or ionization of the air path, with a commensurate severe absorption of the laser energy by the breakdown plasma. Therefore, a single pulse is generally not sufficient for most missions.
When there is no wind to cool and disperse heat in the path of the laser beam, overlap of successive pulses and residual heating from prior pulses cause thermal blooming to be much more of a problem.